How can we measure the coastal currents of Kismayo?

1. Where is Kismayo?

Kismayo is a major coastal city nestled in an area presenting a breathtaking combination of land and sea. It faces the eastern coast of Africa, bordering a big and busy bay. Surrounding the city is a geography that has sandy beaches, mangrove forests in some areas, and plains extending inland.

The local community in Kismayo has a rich maritime heritage; fishing is thus a cornerstone of their local economy and cultural identity. The waters around Kismayo Bay are the habitat of very diverse marine resources. Its crystal-clear waters are full of fish from small reef dwellers to giant pelagic ones. Crabs, lobsters, and all sorts of shellfish are just other invertebrates one may find here. The topography of the seabed consists of sandy areas combined with coral reefs and patches of seagrass. These allow not only the development of different habitats but are also relevant for coastal water movement.

Waters off Kismayo are influenced by larger-scale Indian Ocean currents and the wind from the monsoons. The monsoon season changes the direction and speed of winds, which affects the surface currents accordingly. The ocean currents carry water masses of different temperatures and salinities toward the coast, interacting with the local coastal dynamics. Tides also play a major role in the coastal environment. The ebb and flow of the tides mark the accessibility to shallow areas, the distribution of sediment, and the traffic of boats and ships in the harbor.

2. What is the state of the coastal currents near Kismayo?

A number of factors affect the coastal currents near Kismayo. Dominant among these is the monsoon winds. The winds blow from land to sea during the northeast monsoon and from sea to land during the southwest monsoon, resulting in different surface current patterns. These wind-driven surface currents of seasonal nature interact with the underlying water layers.

In explaining such regular movements of water along a coast, it is important that tides feature prominently. Such a tidal range merely presses the water both ways. This water does indeed move inland when there is an onshore wind-a high tide coupled with strong currents in an onshore manner. Low tides, for their part, make the waters recede with changes in directions of flow. The local topography of coastline and seabed offers equally weighty reasons for this. The shape of the bay near Kismayo can funnel or divert the currents. Coral reefs and mangrove roots will disturb the smooth flow of the water, hence producing areas of turbulence and eddies.

Also, the interaction of coastal currents with the larger Indian Ocean currents is rather complex: oceanic currents approaching the shore can merge with the local currents, thus introducing water masses with different characteristics concerning temperature, salinity, and nutrient content. This might seriously affect productivity in coastal waters and the distribution of marine life.

3. How to Observe the Coastal Water Flow of Kismayo?

Surface Drifting Buoy Method

This method consists of releasing buoys onto the water surface, free to drift with the currents. The position of these buoys, as monitored over time by satellite-based or other positioning systems, provides information on the direction and speed of the surface currents. However, this method is mostly indicative for the surface layer and may not be representative for the currents throughout the whole water column.

Moored Ship Method

A ship is anchored in place in the coastal area, with instruments on the ship measuring flow around it. It can give very detailed information for the area around the ship, but the presence of the ship interferes with the natural flow to some degree, and it can cover only a relatively small area around where it is moored.

Acoustic Doppler Current Profiler (ADCP) Method

ADCP current meter is a more advanced and convenient way of measurement of the coastal currents. It uses acoustic waves in order to simultaneously measure the velocity of water at different depths. That way, it is able to create a detailed profile of the current from the surface down to a certain depth and provide a comprehensive understanding of the structure of the flow of coastal water. It can cover a greater vertical range than the other methods and, in many cases, is less susceptible to external interferences, hence very effective in observing the coastal currents near Kismayo.

4. How do ADCPs using the principle of Doppler work?

Working Principle: The ADCPs are based on the principle of Doppler. They shoot acoustic pulses into the water. These sound waves, while interacting with moving particles in water, such as suspended sediment or small organisms, result in changes to the frequency of the reflected waves due to the Doppler effect. The ADCP current profiler is designed to detect and measure this frequency shift. With multiple transducers oriented in different directions, it calculates the velocity components of the water in various directions, including horizontal and vertical. These obtained profiles can be used for the reconstruction of the three-dimensional flow field of the coastal currents. While recording the reflected signals at specified time intervals, the ADCP flow meter will continuously send out these acoustic pulses. That enables it to generate a time series of current velocity data at varied depths. Hence, it gives a good insight into the manner in which these currents change with both time and depth.

5. What is needed to measure the Kismayo coastal currents at high resolution?

Such measurements of coastal currents around Kismayo require a number of essential features in equipment. First is the material reliability of the equipment. Equipment has to be capable of bearing all the aggressions from the marine atmosphere, such as seawater corrosion, impacts of waves, temperature, and pressure. A compact size will easily enable deployment and handling on different platforms, either boats or buoys.

Other important factors are lightweight, which simplifies installation and reduces the load that is required, and low power consumption, which enables long-term measurements and battery-powered setups. The cost is another important factor to take into consideration with the aim of being able to undertake widespread and large-scale measurements. In this respect, the casing of the ADCP profiler will be made of titanium alloy. The advantages of using a titanium alloy are incomparable. It has excellent corrosion resistance, which enables it to withstand the corrosive nature of seawater for quite a long period without significant degradation. The material is strong, durable, and mechanically resistant to the stresses of water flow and other external impacts. Besides, its relatively low density contributes to maintaining the weight of the equipment at an economic level while sustaining the structural integrity.

6. How to choose proper equipment based on the requirement for current measurement?

Based on Application

• Shipboard ADCP: this would be apt for measurement needed onboard when a ship moves or is stationary in the coastal area. It is able to continuously provide data of the ship when it travels over different locations off the coast and this can be helpful in mapping current patterns over greater areas.
• Bottom-mounted ADCP: Suitable for fixed-point measurements on the ocean floor. By this way, it can correctly monitor the currents that flow above it, providing indications on near-bottom current conditions at the location in question, essential to understand sediment transport and other benthic processes.
• Buoy-mounted ADCP: It has been mounted on buoys floating in the water surface so it is capable of measuring from the surface downward. It provides insight into surface and upper layer current changes thus usually applied if there is a need for long-term observation without the requirement to have any ship present around that area.

Based on Frequency

• A 600 kHz ADCP is normally appropriate for water depths within 70 meters. Due to its higher frequency, it gives a much better resolution in shallow water, with very detailed current information from near the surface and down to the relevant depth range.
• In water as deep as 110 meters, a more appropriate ADCP would be one with a frequency of 300 kHz. This effectively penetrates the water column, measuring currents at different depths within this range.
• For much deeper waters, such as those reaching 1000 meters, the recommended ADCP is a 75 kHz one. Its lower frequency allows it to reach greater depths and still obtain reliable current velocity data.

Some of the popular ADCP manufacturers in the market are Teledyne RDI, Nortek, and Sontek. Still, for clients that require high performance yet affordable means, they can seek a product by China Sonar known as the PandaADCP. It is fully constructed with a metal material called titanium alloy; therefore, one will be assured of outstanding durability in performances in a seawater environment. With its incredible cost-performance ratio, it offers a great choice for measuring the coastal currents of Kismayo. You can learn more about it on its official website: https://china-sonar.com/.

Here is a table with some well known ADCP instrument brands and models.